This invention relates to an apparatus and method for cleaning a semiconductor substrate, and in particular to an apparatus and method for cleaning a semiconductor substrate of sheet form, which make it possible to fully remove even fine particles. Specifically, this invention relates to novel cleaning method consisting of a combination of the conventional contact cleaning (Pen) and non-contact cleaning (CJ) (Cavitation-jet composite pen cleaning method: hereinafter referred to as xe2x80x9cCavipen cleaning methodxe2x80x9d), which enables the flat portion and recessed portion of a cleaning material to be simultaneously cleaned.
As the working dimension in the manufacture of a semiconductor device becomes increasingly finer such as in the order of submicron, even a very fine particle may become a cause for the generation of a defective product, when the particle is allowed to adhere on the surface of a semiconductor substrate to be employed in the production of a semiconductor device. Therefore, it is now demanded to develop a construction schedule control which makes it possible to completely remove such a very fine particle. Further, when a metallic impurity is allowed to adhere onto the surface of a semiconductor substrate, it may become a cause for a failure of electric characteristics even such a metallic impurity is very little in quantity, so that it is also demanded to completely remove such a metallic impurity.
To meet such a demand, there has been recently developed a method for effectively removing particles adhered onto a surface of a substrate, wherein a high pressure pure water impressed with ultrasonic wave of Mega Hertz band is employed to clean the substrate to thereby obtain a very high clean surface of the substrate. An apparatus utilizing such a method is also being developed now.
For example, a sheet cleaning type apparatus utilizing the aforementioned method is known. However, there are a problem that a mist of steam is allowed to generate in the gas phase especially when (1) a high pressure water is jetted against the surface of a semiconductor substrate, or (2) a high-frequency wave (megasonics) is impressed at the occasion of cleaning the substrate by making use of a cleaning nozzle of various type (e.g. a water jet nozzle, a cavitation jet nozzle, megasonic device-attached water jet nozzle, etc.) which is designed to be mounted on the sheet cleaning type apparatus. This mist may be re-adsorbed in a subsequent drying step after the cleaning of a semiconductor substrate, thus rendering the mist to remain as a water mark on the surface of the substrate. Otherwise, this mist may adhere to the dust in the gas phase, thus rendering the mist to be re-adsorbed as a particle on the surface of the substrate. By the way, by the expression of a high pressure water, it means a water having a pressure in the order of several Kgf/cm2 in the case of the megasonic device-attached water jet nozzle, and a pressure in the range of several tens Kgf/cm2 to several hundreds Kgf/cm2 in the case of the a water jet nozzle or a cavitation jet nozzle.
In view of preventing such a mist from being allowed to generate, there has been developed a semiconductor cleaning apparatus which is provided with exhaust ports to be disposed around a semiconductor substrate so as to forcibly discharge the mist.
In the followings, the general structure of the conventional semiconductor cleaning apparatus will be explained with reference to FIG. 34.
FIG. 34 represents a schematic view of the main portion of the conventional sheet form semiconductor substrate cleaning apparatus wherein a semiconductor substrate is placed inside the cleaning apparatus. The reference number 1 denotes the chamber of a semiconductor substrate cleaning apparatus, which is cylindrical in configuration with the top and bottom surfaces thereof being closed. A rod-like substrate holder 2 is disposed inside the chamber 1 in such a manner that it passes through a central portion of the bottom of the chamber 1 while keeping an air-tightness between the bottom of the chamber 1 and the substrate holder 2 and at the same time, ensuring the rotatability of the substrate holder 2 in relative to the bottom of the chamber 1. This substrate holder 2 is connected at one end thereof with a rotating mechanism (not shown) which is disposed outside the chamber 1 thereby enabling the substrate holder 2 to be revolved at a high speed. The other end of the substrate holder 2 which is disposed inside the chamber 1 is connected with a substrate-mounting jig 6. Namely, it is designed such that a semiconductor substrate 20 can be horizontally mounted on the substrate-mounting jig 6 and revolved through the rotation of the substrate holder 2. Further, a nozzle 4 for jetting a high pressure water jet is disposed inside the chamber 1 in such a manner that the tip end 4A of the nozzle 4 is positioned over and slightly spaced away from the top surface of the semiconductor substrate 20. The nozzle 4 is fixingly retained, through a portion near the tip end 4A thereof, by a nozzle frame 5.
On the other hand, a rod-like nozzle-supporting arm 3 is disposed inside the chamber 1 in such a manner that it passes through an upper peripheral surface portion of the chamber 1 while keeping an air-tightness between the upper peripheral surface portion of the chamber 1 and the nozzle-supporting arm 3 and at the same time, ensuring the rotatability of the nozzle-supporting arm 3 in relative to the upper peripheral surface portion of the chamber 1. This nozzle-supporting arm 3 is connected at an upper end thereof with a rotating mechanism (not shown) which is disposed outside the chamber 1 thereby enabling the nozzle-supporting arm 3 to be revolved at a predetermined range of angle. Further, a lower portion of the nozzle-supporting arm 3 which is disposed inside the chamber 1 is L-shaped with the distal end thereof being directed toward the center of the chamber 1 and fixed to the nozzle frame 5.
According to this cleaning apparatus, when the nozzle-supporting arm 3 is rotated to a predetermined angle, the nozzle frame 5 is enabled to scan an entire area along the diametral direction of the semiconductor substrate 20, and when the substrate holder 2 is additionally rotated, the entire surface of the semiconductor substrate 20 can be allowed to come close to the tip end 4A of the nozzle 4.
The nozzle 4 is designed to function also as a feeding pipe for feeding water of high pressure and hence, formed of a flexible tube such as a fine stainless steel tube or a Teflon tube. Further, while ensuring air tightness in relative to the chamber 1, the nozzle 4 is extended out of an upper portion of the chamber 1, leaving a sufficient length thereof inside the chamber 1 so as enable it to follow the rotation of the nozzle-supporting arm 3. One end portion of the chamber 1 is connected with a high pressure water feeding source (not shown) thereby making it possible to continuously feed a high pressure water.
A gas inlet port 10 is formed at a central portion of the upper surface of the chamber 1 thereby making it possible to feed an inert gas such as nitrogen gas into the chamber 1. On the other hand, an exhaust port 11 connected with an outside exhauster (not shown) is formed at a lower portion of the chamber 1 which is lower than the mounting portion of the semiconductor substrate 20, thereby allowing an inert gas fed through the gas inlet port 10 to be discharged from this exhaust port 11. It is possible with this construction to pass an inert gas through the chamber 1 at the occasion of cleaning the surface of the semiconductor substrate 20 by making use of an ejection of a high pressure water, thereby effectively guide and move a mist that has been generated from the high pressure water toward the exhaust port 11 together with the introduced inert gas. At the same time, the high pressure water can be also effectively discharged together with the inert gas.
Next, the method of cleaning the surface of a semiconductor substrate by making use of the aforementioned apparatus will be explained.
First of all, the semiconductor substrate 20 is placed inside the chamber 1, setting it close to the tip end 4A of the nozzle 4 (FIG. 34).
Then, the substrate holder 2 and the nozzle supporting arm 3 are respectively rotated at a desired angle, and at the same time, nitrogen gas is introduced into the chamber 1 from the gas inlet port 10 and discharged from the exhaust port 11 to thereby form a gas flow inside the chamber 1. Then, a high pressure water is allowed to jet from the tip end 4A of the nozzle 4 so as to clean the surface of the semiconductor substrate 20.
In this case, if a high-frequency wave in the order of 1.6 MHz for instance is impressed in advance on the high pressure water, the high-frequency waves will be propagated to the surface of the semiconductor substrate 20 at the moment of jetting a high pressure water, whereby the fine dust that has been adhered onto the surface of the semiconductor substrate 20 would be vibrated and excited, thus causing the fine dust to float upward and enabling the fine dust to be easily removed by the high pressure water. As a result, a clean surface of the semiconductor substrate 20 can be obtained. Although a large quantity of mist may be generated inside the chamber 1 at this moment, since the aforementioned gas flow is formed inside the chamber 1, the mist generated can be discharged together with the gas flow from the exhaust port 11.
Thereafter, the feeding of the high pressure water is stopped, and only the rotation of the substrate holder 2 is continued thereby allowing the semiconductor substrate 20 to dry by the effect of spin-drying.
Next, a cleaning method according to the prior art will be explained.
A cleaning method that has been increasingly employed for the flattening process of semiconductor device after the development of 64 MDRAM and that can be performed using a Chemical Mechanical Polishing (CMP) apparatus is consisted of a flattening (polishing) step and a cleaning step based on a concept of Dry in/out that means that a semiconductor substrate under dry condition is transferred into a CMP apparatus, and the substrate, and the semiconductor substrate is transferred out of the CMP apparatus under dry condition after the completion of planar process and cleaning process therefor.
In the flattening step of wafer, which is a main object of the CMP, chemicals with a polishing particle called slurry are employed. Examples of the polishing particle include alumina (Al2O3), silica (SiO2), ceria (CeO2), etc. The material to be treated by the CMP may be an oxide film, a polysilicon film, tungsten (W), aluminum (Al), copper (Cu). A main object of the cleaning step is to remove the polishing particle employed in the flattening step.
A method of CMP post cleaning which enables to effectively remove the polishing particle remaining on the surface of wafer after the CMP has been also studied by the present inventors. By the way, the CMP post cleaning set forth in the present specification means a cleaning method wherein a wet wafer obtained after finishing the flattening step is spin-dried as it is. Specific examples of such a CMP post cleaning are a roll/sponge cleaning (RS), a pencil sponge cleaning (Pen), a mega-sonic cleaning (MJ), a cavitation jet cleaning (CJ), etc. This CMP post cleaning method currently employed is mainly performed by way of a two-step cleaning wherein the R/S cleaning and the Pen cleaning are sequentially performed, or by way of a three-step cleaning wherein the R/S cleaning, the Pen cleaning and MJ cleaning are sequentially performed.
First of all, problems involved in the employment of the aforementioned conventional semiconductor substrate cleaning apparatus will be explained.
(1) The high pressure water to be jetted from the tip end 4A of the nozzle 4 invites the generation of a large quantity of mist on the surface of the semiconductor substrate 20. Although most of the mist thus generated is discharged from the exhaust port 11, there still remains a relatively large quantity of mist floating in the chamber 1, thereby allowing this floating mist to re-adhere onto the surface of the semiconductor substrate 20. There is a possibility that dust in the gas phase is adsorbed to this mist so that when the water of the mist accompanying the dust is subsequently evaporated, the dust may be left adhered onto the surface of the semiconductor substrate 20, thus making it difficult to remove the dust. Even in the case where mist not accompanying dust is adsorbed to the surface of the semiconductor substrate 20 and then evaporated later, the trace of the mist may be left as so-called water mark.
(2) Since part of mist is left floated inside the chamber as mentioned above, there is much possibility that the mist adhere onto the inner wall of the chamber. Although this inner wall is usually formed of vinyl chloride, if a cleaning liquid containing an acid or an alkali is employed for cleaning a semiconductor substrate, the acid (for example, hydrochloric acid) may react with the alkali (for example, an aqueous solution of ammonia) to form a salt such as ammonium chloride on the surface of the inner wall of the chamber, thus allowing the salt to be left adhered to the inner wall of the chamber as a source for generating particles. Although the aforementioned cleaning liquid is stronger in detergency as compared with pure water, it cannot be employed due to the problems mentioned above.
The following methods have been proposed for solving the above problems.
a) A guard ring type cup is mounted encircling the outer periphery of the semiconductor substrate 20, thereby preventing the generation of the mist which is most likely to be generated from the outer peripheral portion of the semiconductor substrate.
b) A disc having almost the same configuration as that of the semiconductor substrate is placed over the semiconductor substrate, and a solution (liquid) of chemicals is introduced through the central portion of the disc to fill the space between the disc and the wafer with the solution of chemicals to clean the surface of the semiconductor substrate while suppressing the generation of mist.
However, in the case of the method a), although it is possible to prevent the generation of mist around the wafer (semiconductor substrate), it is impossible to prevent mist from generating above the wafer. In the case of the method b), although it is possible to suppress the generation of mist, the essential advantage of water jet which results from the application of ultrasonic wave cannot be obtained.
Next, problems involved in the employment of the aforementioned cleaning method of the prior art will be explained.
FIGS. 35A and 35B illustrate a state where polishing particles remain on the polished surface of a wafer after the flattening step according to the CMP. Specifically, FIG. 35A shows a cross-sectional view of a dishing, FIG. 35B a cross-sectional view of a scratch, and FIG. 35C a cross-sectional view of an alignment marker, wherein the reference numeral 61 denotes polishing particles left remained, the reference numeral 62 a residue of a film that has been once formed on the surface of the wafer but is left remained in a recessed portion, and the reference numeral 63 the wafer.
Depending on the configuration of pattern and the conditions of CMP, the aforementioned recessed portions such as dishing and scratch are caused to be formed on the surface of the wafer, thereby allowing polishing particles to be left remained therein. As another example of such a recessed portion to be formed in the W-CMP, there is an alignment marker (FIG. 3C) which is employed in a photolithography process, thereby creating a possibility that the alignment marker may be clogged with polishing particles. These residual particles are required to be removed.
As for the method for removing these residual particles, there have been proposed a method wherein the residual particles are removed by making use of a physical force and a method wherein the residual particles are removed by making use of a chemical force. As for the former method to remove the residual particles by making use of a physical force, it may be classified into a contact method and a non-contact method.
Among the method to remove the residual particles by making use of a physical force, the contact method (for example, a two-step cleaning consisting of the R/S cleaning and the Pen cleaning) is incapable of directly contacting with the polishing particles buried in a recess, thereby making it very difficult to effectively remove the polishing particles left remained in these dishing, scratch and alignment mark.
There has been also studied to remove the residual particle by means of a non-contact type physical cleaning method such as MJ or CJ. However, in the case of the MJ cleaning, there is a problem that the relationship between the conditions of hardware such as the frequency or output of ultrasonic and the removal ratio of particle alters depending on the dissolved gas concentration of ultrapure water to be employed in the cleaning, thus making it difficult to utilize this MJ cleaning. On the other hand, in the case of the CJ cleaning, there is a problem that a mist of cleaning chemical solution is caused to generate due to the blow-out of high pressure water or due to the vertical ejection of high pressure water against a wafer, thus generating a re-staining by a water mark during the drying step.
Meantime, the assessment of non-contact type physical cleaning such as the MJ or CJ cleaning is generally performed based mainly on the detergency against the particles that have been adsorbed on the surface of a wafer which is flat and free from any recessed portion. Namely, the assessment of detergency against the particles that have been buried in a standardized recessed portion has been scarcely conducted so far.
Therefore, there has been developed a novel cleaning method by the present invention which takes advantage of the conventional contact cleaning (Pen) and non-contact cleaning (CJ) (Cavitation-jet composite pen cleaning method: hereinafter referred to as xe2x80x9cCavipen cleaning method), thus proposing a novel CMP post-cleaning method which makes it possible to simultaneously clean a flat portion as well as a recessed portion of the surface of cleaning material.
Namely, in view of solving the aforementioned problems, the present invention provides a cleaning method of semiconductor substrate, which comprises the steps of:
placing a semiconductor substrate on a substrate holder installed inside a semiconductor substrate cleaning apparatus;
rotating the semiconductor substrate; and
impressing a high-frequency wave on the semiconductor substrate while jetting a high pressure cleaning liquid to a surface to be cleaned (hereinafter referred to also as a cleaning surface) of the semiconductor substrate being kept rotated.
The high-frequency wave to be employed in the aforementioned method should preferably include a high-frequency component of 400 kHz, and also preferably include frequency components continuous within the range of 800 kHz or less. When the high-frequency wave is constituted in this manner, particles of various diameter adhering on the surface of substrate can be effectively removed.
It is also preferable in this method that the high-pressure cleaning liquid is jetted at the cleaning surface from a cleaning nozzle which is mounted movably over the surface of the semiconductor substrate, that the inner diameter of the cleaning nozzle is 0.3 mm or more, and that the distance between the cleaning surface and the cleaning nozzle (a high pressure water blow-out height) is 7 mm or more. The rotating speed of the semiconductor substrate should preferably be not less than 10 rpm, or more preferably 1,000 rpm or more. The pressure to be applied to the high pressure cleaning liquid should preferably be not less than 30 kgf/cm2. It is possible by selecting these conditions in this manner to enhance the cleaning effect where the cavity for generating a high-frequency wave has been effectively formed.
The high-frequency wave including the continuous frequency components may be generated by a single high-frequency generating apparatus comprising a pencil sponge having an open end and a hollow portion connected with this open end, and a cleaning nozzle whose distal end is projected toward the hollow portion. It becomes possible by constructing the high-frequency generating apparatus in this manner to concurrently apply high-frequency waves including various frequency components to the cleaning surface without necessitating the installation of a plural number of high-frequency generating apparatus.
It is also preferable in the aforementioned cleaning method that the pH of the cleaning liquid is selected in such a manner that the zeta potential of the cleaning surface has the same polarity as the zeta potential of the particle to be adhered to the cleaning surface. Alternatively, in addition to the selection of pH of the cleaning liquid, the zeta potential of the particle may be controlled by making use of a surfactant (a cationic surfactant or an anionic surfactant). It is possible by controlling these potentials in this manner to bring about a repulsive force due to the zeta potential difference between the particle adhering to the cleaning surface and the cleaning surface, and hence, to keep the particles away from the cleaning surface or to prevent the particles from re-adhering to the cleaning surface. As a result, the cleaning effect can be enhanced.
Further, in view of solving the aforementioned problems, the present invention provides a substrate cleaning apparatus, which comprises:
a substrate holder for holding a substrate;
a high pressure water-jetting mechanism having a jet nozzle which is directed to face a main surface of the substrate held by the substrate holder;
a chamber housing the substrate holder and the high pressure water-jetting mechanism;
a gas-feeding port communicated with the chamber; and
a gas exhaust port communicated with the chamber;
wherein the high-pressure water-jetting mechanism is composed of a high-pressure water jetting portion and a high-pressure water splash-preventing portion.
The gas-feeding port should preferably be designed such that it is capable of functioning also as a water drainage port.
It is also preferable that the high pressure water jetting portion is constituted by a high pressure water feeding nozzle tip, and the high pressure water splash-preventing portion is constituted by a covering member placed around the high pressure water feeding nozzle tip.
The covering member should preferably be formed of a hollow cylindrical sponge disposed close to the high pressure water feeding nozzle tip and having a cavity formed below the high pressure water feeding nozzle tip.
A substrate cleaning apparatus according to another embodiment of the present invention comprises:
a substrate holder for holding a substrate;
a high pressure water-jetting mechanism provided with an ejection nozzle which is directed to face a main surface of the substrate held by the substrate holder;
a chamber housing the substrate holder and the high pressure water-jetting mechanism;
a gas-feeding port communicated with the chamber; and
a gas exhaust port communicated with the chamber;
and which further comprises a rinsing water feeding pipe for feeding a rinsing water to the other main surface of the substrate.
A substrate cleaning apparatus according to still another embodiment of the present invention comprises:
a substrate holder for holding a substrate;
a high pressure water-jetting mechanism provided with an ejection nozzle which is directed to face a main surface of the substrate held by the substrate holder;
a chamber housing the substrate holder and the high pressure water-jetting mechanism;
a gas-feeding port communicated with the chamber; and
a gas exhaust port communicated with the chamber;
and which further comprises a cleaning tank which is capable of storing a liquid therein and which is housed inside the chamber, the cleaning tank being also capable of housing therein the high pressure water-jetting mechanism and the substrate holder.
Each of the substrate cleaning apparatus should preferably be constructed that the high pressure water-jetting mechanism is provided therein with a high-frequency wave oscillator which is capable of applying high-frequency waves within the range of 300 kHz to 3 MHz to the jetted flow of high pressure water.
The present invention also provides a cleaning method of semiconductor substrate, which comprises the steps of:
placing a semiconductor substrate on a substrate holder installed inside a substrate cleaning apparatus;
housing the substrate holder in a cleaning tank placed inside the substrate cleaning apparatus;
filling the cleaning tank with a cleaning liquid thereby dipping the semiconductor substrate in the cleaning liquid; and
jetting a high-pressure water against one main surface of the semiconductor substrate inside the cleaning tank.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instruments and combinations particularly pointed out hereinafter.